This invention relates to improvements in the composition and method of making an anode for an X-ray tube.
A well known problem in prior art X-ray tubes is that the surface on which the electron beam impinges develops fractures and roughens after many thermal cycles. Surface fractures have a propensity to propogate and sometimes advance until breakage of the target occurs, especially in high speed rotary anode x-ray tubes. Surface fractures allow the electron beam to penetrate such that radiation at the focal spot is intercepted and absorbed by surface layer material. This is manifested in an x-radiation output decrease.
For a long time, anodes or targets as they are sometimes called, were made solely of sintered tungsten of the best purity obtainable. Within about the last decade, laminated anodes were developed comprised of a body of refractory metal such as pure tungsten or pure molybdenum or alloys of these metals and a surface coating for electron impingement comprised of sintered mixtures of tungsten and rhenium powders. The tungsten and rhenium surface layer mixtures have better ductility and lower ductile-to-brittle transition temperatures compared with pure tungsten and exhibited less fracturing after thousands of x-ray exposures.
Tungsten and rhenium surface layer compositions also have reasonably good thermal properties such as high thermal conductivity and low vapor pressure. Use of tungsten-rhenium surface layers does not, however, attain optimum metallurgical properties and fracturing, although reduced in comparison with tungsten or molybdenum alone, is still observed in x-ray tubes which are subjected to the high thermal loading and duty cycles which the most advanced x-ray procedures impose.
One of the residual problems is that the density of the surface layer materials is not close enough to the theoretical maximum density. The inability to approach maximum density means that there are a substantial number of microscopic voids in the surface material. Thermal stresses, due to the intense energy at the focal spot of the electron beam, cause fracture initiation from the surface to the voids located just underneath the surface. Ultimately, the small fractures enlarge and the tube must be removed from service.
Those who are skilled in the metallurgy of x-ray tube anodes appreciate that increasing the density of the anode surface material and reducing the number and size of the voids causes a reduction in fracture initiating sites. It is also understood that if the surface layer material is close to maximum or theoretical density, ductility of the material will be improved since there will be a smaller concentration of voids available to stop dislocation motion. Dislocations must move through the surface layer alloy to relieve stress and prevent fractures. If a moving dislocation encounters a void, it is stopped or arrested and is, therefore, unable to provide additional stress relief. The material will then fracture.
It is known that tungsten can be made more ductile even at room temperature by alloying it with inherently more ductile metals such as rhenium. As indicated above, rhenium has been used for this purpose in x-ray anode surface layers and, to a limited extent, in their bodies or substrates. Rhenium is commonly used as an alloying metal with tungsten but it has the disadvantage of being a very expensive and relatively scarce material. Iridium, rhodium, tantalum, osmium, platinum and molybdenum are further examples of metals which are known to improve ductility when alloyed with tungsten. However, the use of many of these metals in surface layers of high energy x-ray tubes has been avoided because they exhibit high vapor pressures at high temperatures compared with tungsten and are evaporated at peak operating temperatures of the anodes. Some of these metals also have the disadvantages of being relatively expensive and scarce. The evaporated metal deposits on the inside of the x-ray tube envelope and nullifies the insulating properties of the tube so it is less stable at high voltages.
By way of illustration, molybdenum has some properties which make it desirable as an alloy addition to anode surface layers. It has good ductility and susceptibility for being treated metallurgically like tungsten but molybdenum melts at 2610.degree. C compared with tungsten which melts at 3410.degree. C and rhenium which melts at 3180.degree. C. Molybdenum also has an undesirably high vapor pressure, especially at peak anode temperatures existing in the highest power x-ray tubes required today. For example, molybdenum has a vapor pressure of 10.sup.-7 Torr at only 1700.degree. C whereas tungsten has this same vapor pressure at 2260.degree. C and rhenium at 2100.degree. C. Other prospective alloying materials mentioned above and still others have lower melting points and higher vapor pressures than tungsten and they have, heretofore, been considered unqualified as surface layer alloy additions. Of course, as is well known, anodes made solely of molybdenum or molybdenum and tungsten are regularly used in x-ray tubes where abundant soft or low energy radiation is desired such as in tubes used for mammography. These high molybdenum content alloys are, however, restricted to operation at power levels significantly below those required for tubes intended for general diagnostic procedures. As stated earlier, anodes comprised of a molybdenum body with a tungsten-rhenium surface layer are also in widespread use in high energy x-ray tubes but care is taken that none of the molybdenum is permitted near the front surface of the anode in the region of high temperature prevailing at the beam focal spot.
Recently, anodes have been developed which use a graded surface layer. The first outer surface layer on which the electron beam impinges is a tungsten-rhenium alloy. Below the first layer is a second layer which comprises tungsten-rhenium and molybdenum. The content of molybdenum in the second layer diminishes in the direction of the first layer and, conversely, the content of rhenium diminishes in the direction of the substrate which is essentially molybdenum or a molybdenum-tungsten alloy. Thus, no molybdenum from the substrate or the surface layer is exposed to direct electron impact.